They actually are discussed quite frequently, but usually as 'purchase and return' shops.
There are several issues with these. First, in most cases you are doing two rounds of observations for the price of one.
Some of them require driving to the location on different days, while some allow same day. Drive time and gas need to be taken into consideration on the former and on the later, is there something productive you can do in the wait time between the P and the R?
With purchases often in the several hundred dollar range, you need to know your credit card billing cycle because lets say your 'closing date' on your card is the 15th, you make the purchase on the 14th and the return on the 14th but it doesn't get processed until the 17th, the charge without the matching credit will appear on your credit card for payment or interest accumulation.
The sunglasses can work if they are in a mall with either several locations of the client as you do location A, then go down and do location B, then go back to A to make the return, then return to B. But many of the P/R do not work so easily.
Some of the P/R become single report if you decide to keep the item. This works for me because I simply return the item next time I am in the area without the deadline of returning to the area the next day.
For me personally, P/R rarely work because the fee tends to be low, there is no benefit to me for the purchase, any use of my credit card entails some risk and the shops are generally mall based. I tend to avoid the malls because of parking issues and overall low fees for mall shops. Mall parking lots here have more issues in that my vehicle has been 'keyed' in a mall parking lot, someone ran into me while I was parked and left no note, the majority of purse snatchings and vehicle break ins happen in mall parking lots here plus parking is always a pain. To visit a mall to do a $5, $15 and $12 shop on a single visit to me makes very little sense, especially when $100 in damage is done to my vehicle in the parking lot. So I shop shopping centers and strip malls where I have never had an issue in the parking lots.